Our Uplander did this once. I added coolant and it started working fine with no bleeding at all.

 

We shouldn't have to add coolant to the radiator.

The reservoir should be checked regularly and kept topped up. The cooling system should NOT be allowed to suck air from the reservoir.

If your reservoir is empty then there are 2 possible bad things to consider:
1) Your regular maintenance procedure is lacking.
2) You have a leak in your cooling system.

 

As papa bear mentioned, there is a definite cause (loss of coolant) that refilling will solve temporarily. However unfortunately it is most likely due to poor design by GM in 1 of 2 areas (or both and excluding maintenance issues like connections, coolant lines/hoses,etc):

1. Crossover pipe and gasket
2. Head gasket

The crossover pipe has been redesigned during the Uplander's production but my 2008 Cargo does indeed have a small coolant leak at the back of the engine where the head and crossover pipe leak.

Supposedly, the head gasket leaks were contained to the 2007 models (and not all suffered from these) but are possible on later models. A pressure check SHOULD pinpoint the cause.

I'm going to have a pressure test done once i have time to drop the van off at the local GM dealer and go from there. I may even have the crossover pipe/gasket replaced ($250 job) just to eliminate it as the cause if the dealer cannot easily determine the source.